The invention relates to a colour display tube of the "in-line" type comprising in an evacuated envelope an electron gun system for generating three electron beams situated with their axes in one plane. Such a system comprises at least one focusing lens for each beam for focusing the electron beams on a display screen. The focusing lens in the operating display tube is formed by applying a suitable potential difference between first and second lens electrodes which are common to the three electron beams, with lens electrodes each comprise a plate-shaped part which has a central aperture and first and second side apertures situated symmetrically with respect to the central aperture, through which apertures the three electron beams pass.
Such a colour display tube is known from Netherlands Patent Application No. 7 904 114, corresponding to U.S. Pat. No. 4,337,409. The focusing lenses of the integrated electron gun system shown in the above mentioned Patent Application comprises two lens electrodes each consisting of a few electrode components. The two parts of the lens electrodes between which the focusing lenses are formed in the operating display tube by applying a suitable potential difference each consist of a cup-shaped part, the bottom of which has a central aperture and first and second side apertures, through which apertures the electron beams pass. Moreover, each aperture comprises a collar. The diameters of the apertures in the oppositely located parts of the first and second lens electrodes are different.
Colour display tubes are also known, for example, from prior art illustrations in Netherlands Patent Application No. 7 809 160, corresponding to U.S. Pat. No. 4,291,251, in which the distance between the centre of the central aperture and the centre of a side aperture in the first lens electrode is smaller than in the second lens electrode to obtain in this manner static convergence of the three electron beams. This quite generally used method of static convergence, however, results in a substantial beam displacement and/or an asymmetric haze around the spot of the display screen of the outermost electron beams. A better way of obtaining static convergence is described in the above mentioned Netherlands Patent Application No. 7 809 160 corresponding to to U.S. Pat. No. 4,291,251. In the electron gun system described in the patent, the outermost electron beams are deflected in the triode part of the electron gun system and focused symmetrically by tilted focusing lenses. Nevertheless, the sharpness of the spot on the display screen of such tubes often leaves much to be desired. This is the result of astigmatism which results in a non-circular spot or haze around the spot. The astigmatism is caused by errors in the manufacture of the lens electrodes, for example, the non-circularity of the apertures.